After coolant flush, temp gauge seems inconsistent. I have ran car with cap off and attempted to burp the system of air. Top hose gets nice and hot - all seems well and no sign of overheating. Have made several trips and warming/cooling cycles hoping any remaining air would come out.

When driving on the highway, the temp gauge will suddenly go from a normal position and go down to the very low end of the range (almost not giving a reading at all!). After a few minutes it will come back to life, and I seem to notice that it comes back to normal most quickly especially if I turn the heat on.

Any wisdom on what might be happening here? Does it sound more like an air bubble problem or moreso electrical in nature? In general, the temp gauge seems to read lower overall than I recall before I did this coolant flush.

thanks in advance